The production of dry solvents from raw aqueous mixtures is often costly and complicated. The preparation of dry ethanol is a good example. In the conventional process, the raw fermentation broth is stripped under moderate vacuum in a beer still. Overhead vapor from the beer still is sent to a rectification column that produces an overhead product close to the azeotrope (about 93 wt % ethanol) and a bottoms product, which is essentially water. The condensed product from the top of the rectification column is evaporated under pressure and fed to a molecular sieve dryer, which produces ethanol of 99 wt %+ purity. Such a process consumes almost 100 million Btu/h to produce 50 million gallons per year of purified ethanol from a feed containing about 11 wt % ethanol. If the concentration of ethanol in the feed is lower, for example from 0.5 to 5%, the energy consumption of the processes can rise substantially, often exceeding the energy content of the dry ethanol produced.
It is known to use two distillation columns in series to separate mixtures such as organic/water mixtures. Such processes are taught in U.S. Pat. Nos. 4,539,076; 5,035,776; and 7,297,236, for example.
It is also known to use membrane separation to treat the overhead stream from a column. Such processes are taught in U.S. Pat. Nos. 4,978,430 and 7,594,981 and in U.S. Published Application No. 2009/0057128.
In addition, co-owned U.S. Pat. No. 7,732,173 teaches a process for recovering ethanol involving membrane separation, followed by dephlegmation, followed by a second membrane separation step to dehydrate the overhead stream from the dephlegmator.
U.S. Pat. No. 5,105,029 teaches the use of two columns followed by membrane separation, and U.S. Pat. No. 4,405,409 discloses the use of two membrane separation steps in series to treat a column overhead.
Specific membranes for use in dehydration of organic compounds are taught in co-owned U.S. Pat. Nos. 8,002,874 and 8,496,831.
Despite the extensive efforts represented by the prior literature, there remains a need for a process that is both energy efficient and cost effective for producing high purity dehydrated solvents, especially ethanol.